1. Field of the Invention
The present invention relates to a video camera for photographing papers and, in particular, to a video camera which is used in a video apparatus for office automation (OA) such as an electronic image input apparatus and the like.
2. Description of the Related Art
The present invention is used in a video apparatus for OA (office automation) in which a color video camera is mounted to a support member, manuscripts placed on a manuscripts carrying table are photographed, and the video outputs thereof are input to a television monitor or the like.
The above-mentioned OA video apparatus can be used as an overhead proctor (OHP) by connecting the apparatus to a television monitor, to a VTR or to a still image recording apparatus, which records still images on a video floppy disc. Thereby, a filing system is provided which records papers such as pictures, drawings, documents and the like.
The video camera that is used in the above-mentioned 0A video apparatus is an ordinary color video camera which is in principle designed to photograph an ordinary object. For this reason, the color video camera is constructed in such a manner that the .gamma. (gamma) value of a gamma correction circuit for correcting the .gamma. value of a video signal obtained from an image pickup section of the video camera is adjusted by a semi-fixed volume control to be approximately 0.45. Also, the .gamma. value cannot be altered during operation of the apparatus.
When the above-mentioned video camera is used to take a picture of papers, the .gamma. value of the gamma correction circuit is desired to be approximately 0.45 for materials such as pictures (positive images) and the like which are similar to ordinary objects having a good reproducibility of gradation on the screen of a television monitor.
In contrast when the above-mentioned video camera is used to photograph black and white papers and the .gamma. value of gamma correction circuit is set to approximately 0.45 in a similar manner as in the case of the ordinary objects to perform the gamma correction, the characters of the black and white papers on the screen of the television monitor are hinged with white and the characters are hard to see. In this case, if the quantity of light entering the photographing optical system of the video camera is reduced by a stop diaphragm or the like, then the whiteness of the characters can be truly eliminated by the blank blank sections of the papers becoming darker and the characters will be more difficult, to see. This occurs because a certain degree of brightness in the black sections of the papers exists. That is, when the output level of the video camera is set at 100%, the output level of the black sections does not provide 0% but, as shown in FIG. 7, the output level actually provides approximately 40% due to the existence of the brightness so that the brightness level difference between the black and white sections, namely, the contrast therebetween is too small.